Technical Field
The present disclosure is directed at systems, methods, and techniques for transmitting a data frame of an electromagnetic telemetry signal to or from a downhole location.
Description of the Related Art
The recovery of hydrocarbons from subterranean zones relies on the process of drilling wellbores. The process includes drilling equipment situated at surface, and a drill string extending from the surface equipment to a below-surface formation or subterranean zone of interest. The terminal end of the drill string includes a drill bit for drilling (or extending) the wellbore. The process also involves a drilling fluid system, which in most cases uses a drilling “mud” that is pumped through the inside of piping of the drill string to cool and lubricate the drill bit. The mud exits the drill string via the drill bit and returns to surface carrying rock cuttings produced by the drilling operation. The mud also helps control bottom hole pressure and prevent hydrocarbon influx from the formation into the wellbore, which can potentially cause a blow out at surface.
Directional drilling is the process of steering a well from vertical to intersect a target endpoint or follow a prescribed path. At the terminal end of the drill string is a bottom-hole-assembly (“BHA”) which comprises 1) the drill bit; 2) a steerable downhole mud motor of a rotary steerable system; 3) sensors of survey equipment used in logging-while-drilling (“LWD”) and/or measurement-while-drilling (“MWD”) to evaluate downhole conditions as drilling progresses; 4) means for telemetering data to surface; and 5) other control processes such as stabilizers or heavy weight drill collars. The BHA is conveyed into the wellbore by a string of metallic tubulars (i.e., drill pipe). MWD equipment is used to provide downhole sensor and status information to surface while drilling in a near real-time mode. This information is used by a rig crew to make decisions about controlling and steering the well to optimize the drilling speed and trajectory based on numerous factors, including lease boundaries, existing wells, formation properties, and hydrocarbon size and location. The rig crew can make intentional deviations from the planned wellbore path as necessary based on the information gathered from the downhole sensors during the drilling process. The ability to obtain real-time MWD data allows for a relatively more economical and more efficient drilling operation.
Some known MWD tools contain a sensor package to survey the wellbore and send data back to surface using a telemetry method known as electromagnetic (EM) telemetry. EM telemetry involves using an EM telemetry tool to generate EM waves carrying encoded data and transmit these waves from the wellbore through the surrounding formations, and using surface receiving equipment to detect and decode the waves at surface. The BHA metallic tubular is typically used as the dipole antennae for the EM telemetry tool by dividing the drill string into two conductive sections by an insulating joint or connector (“gap sub”) typically placed within the BHA, with the bottom portion of the BHA and the drill pipe each forming an antennae for the dipole antennae. In EM telemetry systems, a very low frequency alternating current is driven across the gap sub. The sub is electrically isolated (“nonconductive”) at the insulating joint, effectively creating an insulating break (“gap”) between the portion of the drill string below the gap and the portion above the gap, which extends all the way up to the surface. The lower part below the gap typically is set as a ground but the polarity of the members can be switched. The low frequency AC voltage and magnetic reception is controlled in a timed/coded sequence to energize the earth and create a measurable voltage differential between the surface ground and the top of the drill string. The EM signal which originated across the gap is detected at surface and measured as a difference in the electric potential from the drill rig to various surface grounding rods located about the drill site.
Typically, a sinusoidal waveform is used as a carrier signal for the telemetry data. The MWD tool comprises a downhole modulator which can use one of a number of encoding or modulation schemes to encode the telemetry data onto a carrier waveform. The three key parameters of a periodic waveform are its amplitude (“volume”), its phase (“timing”) and its frequency (“pitch”). Any of these properties can be modified in accordance with a low frequency signal to obtain a modulated signal. Frequency-shift keying (“FSK”) is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave. The simplest FSK is binary FSK (“BFSK”). BFSK uses a pair of discrete frequencies to transmit binary information. Amplitude shift keying (“ASK”) conveys data by changing the amplitude of the carrier wave; phase-shift keying (“PSK”) conveys data by changing, or modulating, the phase of a reference signal (the carrier wave). It is known to combine different modulation techniques to encode telemetry data.
The choice of modulation scheme uses a finite number of distinct signals to represent digital data. PSK uses a finite number of phases, wherein each phase is assigned a unique pattern of binary digits, or “symbols”, and wherein the symbols together form a defined symbol set. Usually, each phase encodes an equal number of bits. Each pattern of bits forms the symbol that is represented by the particular phase. A surface demodulator contains the same symbol set used by the downhole modulator, and determines the phase of the received signal and maps it back to the symbol it represents, thus recovering the original data.